Antisymmetric exchange interactions in Ni₄ clusters

We show that in molecular Ni₄ clusters with S₄ symmetry, not only the local anisotropy but also the antisymmetric exchange interaction contributes to the observed fourth-order anisotropy terms in the single-spin Hamiltonian. From a group-theoretical classification of the exchange-coupled multiplets, the effective spin Hamiltonian terms are determined. We find that low-lying excited spin states possess an orbital angular momentum, which mixes into the spin ground state, leading to an effective antisymmetric exchange term. The simulation of the magnetic and spectroscopic experimental data allowed the extraction of parameter values for all interactions.